This invention relates to an image forming apparatus such as a copier and a laser beam printer, and in particular relates to an image forming apparatus for conducting a transfer output control operation.
The schematic structure of conventional image forming apparatuses will be described with reference to FIG. 7. Such an image forming apparatus has image forming devices 10Y, 10M, 10C, and 10K which form the images of the colors yellow, magenta, cyan and black respectively, and these have photoreceptor drums 1Y, 1M, 1C and 1K respectively which are image bearing members, and each photoreceptor drum rotates in the direction of the arrow (anticlockwise). Charging devices 2Y, 2M, 2C and 2K, exposing devices 3Y, 3M, 3C and 3K, developing devices 4Y, 4M, 4C and 4K, and cleaners 8Y, 8M, 8C and 8K are sequentially arranged on the periphery of the photoreceptor drums 1Y, 1M, 1C and 1K respectively in the direction of rotation thereof. The images formed on the photoreceptor drum using the developing agent in each image forming device are sequentially transferred by each of the first transfer rollers 7Y, 7M, 7C and 7K which are transfer devices, onto the belt-like intermediate transfer member 6 which moves and passes adjacent to the photoreceptor drum. The images that are transferred onto the intermediate transfer member 6 are further transferred to the recording material P such as paper at the second transfer roller 73.
In this image forming apparatus, such changes as in the properties of the transfer roller and the intermediate transfer member that are used, the physical properties of the toner, and the properties of the photoreceptor due to environmental changes and the passage of time sometimes cause changes in image density. Such an image forming apparatus generally has a mechanism for adjusting image density, and many have devices which automatically adjust their image density to an optimum level. In particular, in an image forming device which performs full color image output, more accurate control of yellow, magenta, cyan and black respectively is required in order to obtain a desirable color balance. Examples of the background technology are described in the following.
A technology has been disclosed wherein the transfer bias is obtained from the developing bias value and control is thereby performed. More specifically, the developing bias is obtained based on the density of the toner patch image, and also the relationship between the developing bias and the transfer bias is determined in advance and the transfer bias is obtained using this relationship, from the obtained developing bias (See Patent Document 1).
A technology has been disclosed wherein, in the case where control is conducted by increasing or decreasing the charge amount per unit area of the toner image on the image bearing member, the transfer bias is subsequently reset by the transfer bias setting device (See Patent Document 2).
A technology has been disclosed in which the toner patch image is formed on the image bearing member, and the transfer bias is determined based on detection of the density of the toner patch image that was transferred onto the intermediate transfer member from the image bearing member (See Patent Document 3).
[Patent Document 1]                Tokkai 2002-244369 (Japanese Non-Examined Patent Publication)        
[Patent Document 2]                Tokkai 2003-241544 (Japanese Non-Examined Patent Publication)        
[Patent Document 3]                Tokkaihei 09-218598 (Japanese Non-Examined Patent Publication)        
However, the background technologies described above have the following problems.
In Patent Document 1, it is known that properties such as of the transfer roller and the intermediate transfer member change due to environmental changes and the passage of time. When these changes occur, the aforementioned relationship that is obtained in advance between the developing bias and the transfer bias sometimes changes, and thus a problem arises in that it is difficult to accurately determine the transfer bias.
In Patent Document 2, the charge amount per unit area of the toner image on the image bearing member must be adjusted, and also the transfer output must be set and thus control is difficult. In addition, even when the transfer output is reset, it is simply obtained from the relationship data for voltage/current values, and the density of the actual toner image is not detected and thus there is a problem in that obtaining accurate transfer output is difficult.
In the Patent Document 3, when the transfer output value is adjusted, if the developing bias or the laser beam intensity or other values change, it becomes impossible to keep the density of the toner image on the intermediate transfer member within a fixed range, and there is a problem in that in this state, from the density of the toner patch image that was transferred to the intermediate transfer member, obtaining accuracy transfer output is difficult.